1. Field of the Invention
The present invention relates to a color still picture transmission system. More specifically, the present invention relates to a color still picture transmission system in which luminance signal data and two kinds of color difference signal data of a color still picture is transmitted between color visible telephones, for example, by using a communication line such as a subscriber's line.
2. Description of the Prior Art
In the Japanese Patent Application Laying-open No. 154887/1980 laid-open on Dec. 2, 1980, a color still picture transmission system capable of transmitting a color still picture in a relatively short time period is disclosed. In this prior art, a luminance signal Y and color difference signals I and Q of a color picture taken by a color TV camera are respectively converted into PCM digital data. At this time, the I and Q signals are digitally converted with a converting cycle of 1/6 of the converting cycle for the Y signals. The digital data of the Y, I and Q signals are stored in a memory. The Y signal data are read from the memory during a period equal to three lines, and thereafter, the I signal data and the Q signal data which are band-reduced to 1/6 are continuously read from the memory during a period equal to one line of the Y signal data, being transmitted. At the receiving party, respective data of the Y signal, and the I signal and the Q signal which are sent from the sending party are written in a memory. After that the Y signal data of three lines are read from the memory, the I signal data and the Q signal data which are band-reduced to 1/6 are continuously read from the memory, respectively, during a period equal to three lines of the Y signal data.
In accordance with this prior art, since a color still picture can be transmitted in a transmission time period of approximately 4/3 of a time period necessary for transmitting a monochromatic still picture, there is an advantage that substantial shortening of the transmission time can be expected in comparison with a case where the digital data of the Y, I and Q signals are transmitted for each picture element.
However, in the above described prior art, since the I signal and the Q signal of a single picture element are merely sampled at every six picture elements of the Y signal, there is a disadvantage that such color difference signal data is susceptible to a sampling noise. The reason is that if a noise accidentally exists in the color difference signal in sampling the same, the noise level is undesirably sampled and thus the noise level is deemed as a correct value of the I signal or Q signal.
In addition, in the above described prior art, since the data of the Y, I and Q signals are transmitted from the sending party every three lines, there is a further disadvantage that a long time is required until a whole screen is completely displayed at the receiving party.